1. Field of the Invention
The present invention relates, generally, to a clutch assembly, and more specifically, to a start-up clutch and torsional-vibration damper assembly for translating torque between a prime mover and a transmission.
2. Description of the Related Art
Generally speaking, land vehicles require a powertrain consisting of three basic components. These components include a power plant (such as an internal combustion engine), a power transmission, and wheels. The power transmission component is typically referred to simply as the “transmission.” Engine torque and speed are converted in the transmission in accordance with the tractive power demand of the vehicle. Hydrokinetic devices, such as torque converters, are often employed between the internal combustion engine and its associated automatic transmission for transferring kinetic energy therebetween.
Torque converters typically include impeller assemblies that are operatively connected for rotation with the torque input from an internal combustion engine, a turbine assembly that is fluidly connected in driven relationship with the impeller assembly, and a stator or reactor assembly. These assemblies together form a substantially toroidal flow passage for kinetic fluid that circulates in the torque converter. Each assembly includes a plurality of blades or veins that act to convert mechanical energy to hydrokinetic energy and back to mechanical energy. The stator assembly of a conventional torque converter is locked against rotation in one direction but is free to spin about an axis in the direction of rotation of the impeller assembly and the turbine assembly. When the stator assembly is locked against rotation, the torque is multiplied by the torque converter. During torque multiplication, the output torque is greater than the input torque for the torque converter. However, when the stator assembly freewheels in the direction of rotation of the impeller and turbine assemblies, there is no torque multiplication and the torque converter becomes a fluid coupling. Fluid couplings have inherent slip. In the absence of a fully engaged lock-up clutch, torque converter slip exists when the speed ratio is less than 1.0 (RPM input>RPM output of the torque converter). This inherent slip reduces the efficiency of the torque converter.
While torque converters provide a smooth coupling between the engine and the transmission, the slippage of the torque converter results in parasitic losses that decrease the efficiency of the entire power train. More specifically, the operating efficiency of the converter during start-up is relatively low. It varies from a zero value at stall to a maximum value of approximately 80-85% at the coupling point. The coupling point occurs at the transition from the torque multiplication mode to the coupling mode when the torque multiplication ratio is unity.
In addition to the problems with efficiency, torque converters of the type known in the related art occupy substantial space in the driveline assembly between the transmission gearing and the engine. Torque converters typically define relatively large diameters when compared to the transmission gearing. Further, the torque converter has a substantial rotating mass that must be accelerated by the engine during start-up of the vehicle during forward drive or in reverse drive. The effective mass of the converter necessarily includes the mass of the hydraulic fluid that circulates in the torus circuit defined by the converter impeller, the turbine, and the stator assembly.
On the other hand, frictional clutches have been also employed in the related art to selectively connect a source of rotational power, such as the crank shaft of an internal combustion engine and its flywheel, to a driven mechanism, such as a transmission. The frictional clutches of the type that have been employed in the related art overcome the disadvantages associated with reduced efficiencies, parasitic losses, relatively large effective mass and the space that is occupied by torque converters used for the same purpose. In an automotive context, clutches used for this purpose are often referred to as “start-up” clutches. Clutches of this type typically include a clutch pack that is operatively supported between a drive and driven member of the clutch assembly. The drive member is operatively connected to the torque input from the prime mover. The driven member is operatively connected to the input shaft of the transmission.
In addition, some start-up clutches include a series connected, torsional-vibration dampers incorporated between the clutch pack and the output to the input of the transmission. A torsional-vibration damper is, generally, a type of elastic coupling disposed between the two main components of drive train of a vehicle (i.e., the engine and the transmission). Such devices reduce or otherwise prevent vibrations from being transmitted from the engine to other parts of the drive train.
The basic embodiment of a torsional-vibration damper includes a primary element and a secondary element that are coupled to each other through a spring dampening device and are limited in movement in relation to each other about a rotational axis. The spring device advantageously includes a plurality of springs disposed on a radial arc spaced relative to the rotational axis and, preferably, at a certain uniform distance from each other. The springs, or a sequence of multiple springs, if applicable, are then connected to the primary element on one side of the springs and to the secondary element on the other side of the springs. Torque is transmitted through the spring coupling, and, as a result of the spring characteristic, a certain additional damping effect is achieved.
In contrast to the aforementioned torque converters, combining a start-up clutch with a torsional vibration damper provides the additional advantages of smoother operation and ride characteristics for the vehicle, while reducing impact loading on driveline components thereby extending the life of the components. However, in conventional applications, the combination of a start-up clutch and a torsional vibration damper loses most of the space saving and mass reduction advantages of the start-up clutch alone versus a torque converter. Up to this point, the trade off of mass and space taken up when employing a combined start-up clutch and torsional vibration damper has been tolerated. However, as the progression toward lighter, smaller, and more efficient vehicle driveline components continues, the general size and mass of the conventional start-up clutch and torsional-vibration damper has become less tolerable and has proven to be a limitation to greater drive line efficiency.
Furthermore, early attempts at producing more compact start-up clutch and torsional-vibration damper assemblies have found that a smaller, more compact device must deal with larger frictional loads and thereby much higher heat dissipation to transfer the same torque loading as conventional devices. Thus, to this point, attempting to achieve a higher level of efficiency by reducing component size and weight of combined start-up clutch and damper assemblies has been problematic due to the excessive heat that is generated and must be dissipated.
Accordingly, there remains a need in the related art for a start-up clutch and torsional vibration damper assembly that is radially and axially more compact than conventional designs to provide more efficiency and cost savings by occupying less space and having less weight than conventional designs. Furthermore, there remains a need in the art for a more compact and lighter start-up clutch and a torsional vibration damper assembly that is capable of dissipating the higher frictional heat output normally generated by these devices.